Recently, there is an increasing interest in energy storage technology. Batteries have been widely used as energy sources in portable phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development into them. In this regard, electrochemical devices are subjects of great interest. Particularly, development of rechargeable secondary batteries is the focus of attention. Recently, research and development into novel electrode and battery that can improve capacity density and specific energy have been developed intensively in the field of secondary batteries.
Among currently used secondary batteries, lithium secondary batteries appearing in early 1990's have drive voltage and energy density higher than those of conventional batteries using aqueous electrolytes (such as Ni-MH batteries, N—Cd batteries, H2SO4—Pb batteries, etc). For these reasons, lithium secondary batteries are advantageously used. However, such lithium secondary batteries have disadvantages in that organic electrolytes used therein may cause safety-related problems resulting in ignition and explosion of the batteries and that processes for manufacturing such batteries are complicated. The latest lithium ion polymer batteries improve the above disadvantages of lithium ion secondary batteries and are expected to be the most prominent candidate leading the next-generation battery technology. However, lithium ion polymer batteries generally have lower battery capacity compared to lithium ion secondary batteries. Particularly, they have insufficient discharge capacity at low temperature. Therefore, there is a need of improvement in such problems.
It is very important to evaluate and secure the battery safety. The most important consideration is that batteries should not cause damages to users upon miss-operation of the batteries. For this purpose, safety standards for the batteries strictly restrict firing and explosion in the batteries. Thus, many methods to solve the battery safety problem are being proposed.
Particularly, as a more fundamental solution for battery safety, use of polymer electrolytes has been suggested. Generally, battery safety increases in order of liquid electrolytes, gel-type electrolytes and solid polymer electrolytes, while battery performance decreases in the same order as described above. Therefore, it is known that batteries using solid electrolytes have not yet been commercialized due to such poor battery performance. Meanwhile, commercially applicable gel-type polymer electrolytes are recently developed by Sony Corp. and Sanyo Electric Co. Ltd. in Japan, and are disclosed in U.S. Pat. No. 6,509,123 B1 and Japanese Laid-Open Patent No. 2000-299129, respectively. Batteries using such gel-type polymer electrolytes are also produced. The characteristics of the above-mentioned two types of batteries will be described hereinafter briefly.
The Sony's batteries use a polymer such as PVDF-HFP (polyvinylidene fluoride-hexafluoropropylene) and an electrolyte containing LiPF6 dissolved in EC (ethylene carbonate) and PC (propylene carbonate). The polymer and electrolyte are mixed with DMC (dimethyl carbonate) as a solvent to form a mixture, and then the mixture is coated on the surface of an electrode, followed by volatilization of DMC, thereby providing an electrode having gel-type polymer thereon. Next, the electrodes are wound together with a polyolefin-based separator for preventing electric short circuit, thereby providing a battery.
Meanwhile, in the case of the Sanyo's batteries, a cathode, an anode and a polyolefin-based separator are wound to form a cell, in the first place. Then, PVDF (polyvinylidene fluoride), PMMA (polymethyl methacrylate), PEGDMA (polyethyleneglycol dimethacrylate) and an initiator are blended with a mixture of organic carbonates. Next, the resultant mixture is injected into the previously formed cell and then is crosslinked under suitable conditions, thereby forming a gel-type polymer electrolyte. In this case, the in-situ formation of the electrolyte is made inside of a battery after assemblage of a battery.
However, it is shown that processes for preparing the above two types of gel-type polymer electrolytes are very complicated and thus provide poor productivity. Moreover, the above two-types of electrolytes result in only limited improvement in battery performance and safety.
As another approach to improve battery safety, Korean Patent Publication No. 0366344 discloses a method of coating a conductive polymer on the surface of an electrode active material. However, in this case, there are problems in that electrode active material particles coated with conductive polymers tend to aggregate, and that the conductive polymers may be separated from the surface of the electrode active material due to the solvents and shear force used in the following steps for manufacturing an electrode. Additionally, although conductive polymers permit electron movement, lithium ion movement is highly limited, thereby causing degradation of battery performance.